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Possible Duplicate:
C++ string::find complexity
What is the time complexity of the find operation that comes built-in with the string library in STL?
The Standard, §21.4.7.2, doesn’t give any guarantees as to the complexity.
You can reasonably assume
std::basic_string::findtakes linear time in the length of the string being searched in, though, as even the naïve algorithm (check each substring for equality) has that complexity, and it’s unlikely that thestd::stringconstructor will build a fancy index structure to enable anything faster than that.The complexity in terms of the pattern being searched for may reasonably vary between linear and constant, depending on the implementation.